Impregnation of wood chips prior to the manufacture of pulp therefrom, either chemically or semi-chemically, or prior to producing pulp mechanically from said chips, for instance in a disc refiner, can be made easier and improved by compressing the chips between rotating rollers. See CA 773,835. The wood chips preferably contain 40-55 percent by weight water and are compressed to a thickness of from 1/5-1/10 of their original thickness in the roll nip defined between a pair of rollers. The rollers act on the mutually opposite flat sides of the chips to loosen the bonds between the fibres and to render the chips more porous. The chips return almost to their original shape subsequent to compression.
Thicker chips are compressed more thoroughly than thinner chips between the rollers. This results in an increase in the porosity of the thicker chips, so as to render the entire cross-section of the thicker chips accessible to an impregnating liquid and to achieve comparatively uniform impregnation. The thinner chips do not benefit from as high a degree of compression and have a lesser increase in porosity. Infeed of the chips into the roll nip can be made easier by roughening the roller surfaces.
WO 89/02951 relates to improving uniformity during the impregnating process. This is accomplished by sawing mutually identical wood pieces having a length of, e.g., 100 mm in the fibre direction and a width and thickness of 40.times.10 mm transversely to the fibre direction, and compressing the wood pieces at right angles to the fibre direction prior to impregnation. For instance, the wood pieces can be compressed between rollers or press plates which act on the mutually opposite flat sides of said wood pieces. When rollers are used, the rollers may be shallowly serrated or fluted, so as to facilitate movement of the wood pieces into the roll nip.